Motor-driven actuator with hydraulic force amplification

ABSTRACT

The present invention provides an actuator ( 20 ) for selectively displacing an output member ( 33 ) against an opposing load (L) in response to the output ( 24 ) of a motor ( 23 ). The actuator includes a first transmission mechanism ( 50 ) for displacing the output member relative to the motor at a first nominal ratio with respect to the motor output; a second transmission mechanism ( 60 ) for displacing the output member relative to the motor at a nominal second ratio with respect to the motor output; a selector ( 34, 39 ) for coupling the motor output to the output member only through the first transmission mechanism ( 50 ) when the load is less than a predetermined value, and for coupling the motor output to the output member through the first and second transmission mechanisms ( 50, 60 ) when the load is greater than the predetermined value. The force exerted by the output member on the load is the sum of the forces transmitted through the first and second transmission mechanisms.

TECHNICAL FIELD

[0001] The present invention relates generally to the field of actuators for moving a load, and, more particularly, for an improved two-stage motor-driven actuator with a ball-screw first stage and a hydraulic second stage, which is particularly useful in selectively opening and closing an injection mold used in plastic molding.

BACKGROUND ART

[0002] Plastic molding machinery typically requires actuators which can provide both rapid motion at low force through a relatively long stroke, followed by application of high force through a short stroke. To meet these requirements with electric motor drives requires excessively large motors to provide the high force needed, or a non-linear toggle mechanism to approximately match the performance of a smaller motor to the load requirements. The present invention avoids these alternatives by effectively introducing an “automatic transmission” on the motor drive.

[0003] One type of mechanical mold clamping mechanism is shown and described in U.S. Pat. No. 4,968,239, the aggregate disclosure of which is hereby incorporated by reference. Briefly, this reference appears to disclose the use of either a high-speed low-force transmissive path or a low-speed high-force transmissive path. However, these paths are used alternatively and not additively.

DISCLOSURE OF THE INVENTION

[0004] With parenthetical reference to the corresponding parts, portions or surfaces of the disclosed embodiment, merely for purposes of illustration and not by way of limitation, the present invention broadly provides an improved actuator (20) for selectively displacing an output member (33) against an opposing load (L) in response to the output of a motor (23).

[0005] The improved actuator (20) broadly includes a first transmission mechanism (50) for displacing the output member (33) relative to the motor (23) at a nominal first ratio with respect to the motor output; a second transmission mechanism (60) for displacing the output member relative to the motor at a second nominal ratio with respect to the motor output; a selector (34,39) for coupling the motor output to the output member only through the first transmission mechanism when the load is less than a predetermined value, and for coupling the motor output to the output member through the first and second transmission mechanisms when the load is greater than the predetermined value; and wherein the force exerted by the output member on the load is the sum of the forces transmitted through the first and second transmission mechanisms.

[0006] In the preferred embodiment, the force exerted by the first transmission mechanism on the output member is substantially limited to the predetermined value. The selector preferably couples the motor output to the output member through the second transmission mechanism only when the load exceeds the predetermined value. The displacement range of the second transmission mechanism is preferably less than the displacement range of the output member, and the second transmission mechanism may be provided with a clutch capable of selectively connecting the second transmission mechanism to the output member at an point in the output displacement range. This clutch may be configured and arranged to automatically connect the second transmission mechanism to the output member whenever the load is greater than the predetermined value. The first transmission mechanism may include a spring (39) preloaded to the predetermined value. The first transmission mechanism may include a ball-screw (25) and nut (34), and the second transmission mechanism may include a hydrostatic coupling. In the preferred embodiment, the motor (23) is electrically powered, and the motor output is a rotatable shaft on the motor.

[0007] Accordingly, the general object of the invention is to provide an improved actuator for selectively displacing an output member against an opposing load.

[0008] Another object is to provide an improved actuator that is particularly suited for use in closing mold halves in plastic molding machinery.

[0009] Another object is to provide a two-stage actuator in which an output member is movable quickly through a displacement range at low force, and can be thereafter moved more slowly at a higher force, as when it is desired to pack a mold.

[0010] These and other objects and advantages will become apparent from the foregoing and ongoing written specification, the drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a fragmentary schematic vertical sectional view of the improved actuator.

DESCRIPTION OF THE EMBODIMENTS

[0012] At the outset, it should be clearly understood that like reference numerals are intended to identify the same structural elements, portions or surfaces, consistently throughout the several drawing figures, as such elements, portions or surfaces may be further described or explained by the entire written specification, of which this detailed description is an integral part. Unless otherwise indicated, the drawings are intended to be read (e.g., cross-hatching, arrangement of parts, proportion, degree, etc.) together with the specification, and are to be considered a portion of the entire written description of this invention. As used in the following description, the terms “horizontal”, “vertical”, “left”, “right”, “up ” and “down”, as well as adjectival and adverbial derivatives thereof (e.g., “horizontally”, “rightwardly”, “upwardly”, etc.), simply refer to the orientation of the illustrated structure as the particular drawing FIGURE faces the reader. Similarly, the terms “inwardly” and “outwardly” generally refer to the orientation of a surface relative to its axis of elongation, or axis of rotation, as appropriate.

[0013] Referring know to the drawing, the present invention broadly provides an improved actuator, of which a presently-preferred form is generally indicated at 20.

[0014] Actuator 20 is shown as being mounted on a suitable support 21. More particularly, a horizontal cylindrical guide rod 22 extends rightwardly from the vertical surface of support 21 for purpose hereinafter explained. An electric motor 23 has its stator or casing mounted on support 21, and has a rotatable output shaft 24 connected to a ball-screw 25 which extends rightwardly therefrom along an axis (x₁-x₁) parallel to the axis (x₂-x₂) of guide rod 22.

[0015] The actuator is shown as having a specially-configured body, generally indicated at 26. This body has a lower blind recess 28 to receive and accommodate the ball-screw, an upper chamber 29 provided with aligned openings at either end to accommodate passage of an intermediate portion of guide rod 22, an internal chamber 30, and a first passageway 31 communicating chamber 30 with upper chamber 29 and a branch passageway 32 communicating passageway 31 with lower chamber 28. The body is shown as having an eye 33 which functions as an output member. More particularly, an external load L is shown as being applied to this eye 33.

[0016] A piston-like member 34 is operatively arranged within chamber 28 for axial movement therealong. Piston 34 has an inner nut that is arranged to matingly engage the external threads on ball-screw 25. Additionally, piston 34 has an outer flange that is sealed to the recess walls by O-rings 35 and 36 to form annular hydraulic pressure area A₁. Hence, chamber 38 is sealed, and communicates via passage way 32 with passageway 31. The piston is preloaded to move leftwardly relative to the body against a shoulder in chamber 28 by means of a spring 39.

[0017] A bellows-like reservoir 40 is operatively arranged in chamber 30 and communicates with passageway 31 via a check valve 41. Check valve 41 is configured to permit fluid to flow from the interior of bellows 40 to passageway 31, but to prevent flow in the opposite direction.

[0018] A clutch assembly, generally indicated at 42, is operatively arranged in upper chamber 29. This clutch assembly includes an annular drive piston 43 surrounding guide rod 22, a plurality of collet wedges 44, a collet ring 45, a Belleville spring stack 46 acting between the body and the collet ring, and a coil spring 48 acting between the collet ring and the collet wedges. Belleville spring stack 46 urges collet ring 45 to move rightwardly relative to the body until the right marginal end of collet ring 45 abuts an annular stop 49 extending inwardly into the chamber from the body. Spring 48 urges the collet wedges to move rightwardly relative to the collet ring, in tight abutting relation to the left end of the drive piston. Springs 46 and 48 also urge the drive piston to bottom on the leftwardly-facing surface of upper body chamber 29. Piston 43 is sealed to the body by O-ring 50, and is sealed to the guide rod by O-ring 51, thereby forming annular hydraulic pressure area A₂. Fluid in chamber 38 continuously communicates with the space 52 between the right end face of piston 43 and the body by means of communicating passageways 31 and 32.

[0019] The ball-screw 25 and nut 34 comprise a first transmission mechanism, generally indicated at 50, for displacing the output member 33 relative to the motor at a nominal first ratio with respect to the motor output. In other words, when the load is less than a predetermined value established by the preload of spring 39, piston/nut 34 is held in contact with body 26 and the axial movement of the output member will therefore be a function only of the lead of the ball-screw 25. Hence, the output member will be displaced in the appropriate direction by a certain distance per revolution of the motor output shaft. The second transmission mechanism, generally indicated at 60, includes the clutch assembly 42. Basically, as long as the load L acting on the output member is less than the predetermined value, motion of the output member is governed solely and exclusively by the ball-screw, and the second transmission means is inactive. In this regard, the force exerted by spring 39 on piston 34 sets a predetermined maximum value of the force exerted through the first transmission means. However, should the applied load L exceed the force exerted by spring 39, then continued rotation of the ball-screw will cause piston 34 to move rightwardly relative to the body, decreasing the volume of chamber 38, and forcing fluid to flow through communicating passageways 32 and 31 into chamber 52. This in turn drives piston 43 leftwardly and will cause the collet wedges to tightly engage the guide rod. Thereafter, continued flow displacement of fluid from chamber 38 through passageways 32, 31 will cause displacement of piston 43 relative to the body, thereby moving the body relative to the point at which the collet wedges 44 engaged the guide rod 22. Since the area A₁ of piston/nut 34 is designed to be substantially smaller than the area A₂ of piston 43, motion of the body when driven through the second transmission mechanism will be much less per motor revolution than when driven directly by the ball-screw alone. At the same time, the force developed by the motor and screw will be multiplied by the piston area ratio, A₂/A₁.

[0020] Recapitulating, when the applied load is less than the preload force exerted by spring 39, rotation of the motor will cause the output member to be translated, either leftwardly or rightwardly as desired, solely by the ball-screw/nut mechanism. In this condition, the clutch is not activated, and simply slides along the guide rod. However, when the applied load exceeds the force exerted by spring 39, piston 34 moves relative to the body, forcing fluid from chamber 38 into chamber 52. This then causes the drive piston 43 to move leftwardly, activating the clutch, and causing additional force to be exerted on the body and output member. The output stroke over which this additional force can be applied is limited to the stroke of the piston 34 divided by the area ratio A₂/A₁. The total force exerted on the output member is therefore the hydraulic pressure force plus the spring preload force, is the sum of the forces transmitted through the first and second transmission means.

[0021] Modifications

[0022] The present invention contemplates that many changes and modifications may be made. For example, in the preferred embodiment, motor 23 is shown as being an electrically-powered motor having a rotatable output shaft. However, this motor is not limited to this form, and could include other forms as well. For example, motor 23 could be pneumatically or hydraulically operated. In another configuration, the motor could have a linear output member, as opposed to a rotatable shaft. Similarly, the particular structure of the clutch mechanism may be readily changed as desired. Springs other than Belleville springs could be used in lieu thereof.

[0023] Therefore, while the presently preferred form of the improved actuator has been shown and described, and several modifications thereof discussed, persons skilled in this art will readily appreciate that various additional changes and modifications may be made without departing from the spirit of the invention, as defined and differentiated by the following claims. 

What is claimed is:
 1. An actuator for selectively displacing an output member against an opposing load in response to the output of a motor, comprising: a first transmission mechanism for displacing said output member relative to said motor at a nominal first ratio with respect to said motor output; a second transmission mechanism for displacing said output member relative to said motor at a nominal second ratio with respect to said motor output; a selector for coupling said motor output to said output member only through said first transmission mechanism when said load is less than a predetermined value, and for coupling said motor output to said output member through said first and second transmission mechanisms when said load is greater than said predetermined value; and wherein the force exerted by said output member on said load is the sum of the forces transmitted through said first and second transmission mechanisms.
 2. An actuator as set forth in claim 1 wherein the force exerted by said first transmission mechanism on said output member is substantially limited to said predetermined value.
 3. An actuator as set forth in claim 2 wherein said selector couples said motor output to said output member through said second transmission mechanism only when said load exceeds said predetermined value.
 4. An actuator as set forth in claim 1 wherein the displacement range of said second transmission mechanism is substantially less than the displacement range of said output member, and wherein said second transmission mechanism is provided with a clutch capable of selectively connecting said second transmission mechanism to said output member at any point in said output member displacement range.
 5. An actuator as set forth in claim 4 wherein said clutch is configured and arranged to automatically connect said second transmission mechanism to said output member whenever said load is greater than said predetermined value.
 6. An actuator as set forth in claim 2 wherein said first transmission mechanism includes a spring preloaded to said predetermined value.
 7. An actuator as set forth in claim 1 wherein said first transmission mechanism includes a ball-screw and nut.
 8. An actuator as set forth in claim 1 wherein said second transmission mechanism includes a hydrostatic coupling.
 9. An actuator as set forth in claim 1 wherein said motor is electrically powered.
 10. An actuator as set forth in claim 1 wherein said motor output is a rotatable shaft. 